February 1-7, 2005
Midwest Weekly Highlights - February 1-7, 2005
Mild Start to February
The first week of February was generally mild and quiet across the
Midwest. Precipitation the first five days of the month was
confined to the far southern portions of the region
(Figure 1).
The dry weather allowed rivers and streams in the
southern half of the Midwest to continue to fall after reaching flood
levels due to the copious January precipitation. A low pressure system
moving along the Gulf Coast with a trough extending northward on
February 2nd brought rain to an area from southern Missouri
through southern Illinois, Indiana, and Kentucky. Additional
rain came on February 6-7 as a weak cold front dropped south through
the Midwest. Precipitation for the week was above normal from
western Missouri and Iowa northeast through Wisconsin and northern
Michigan, largely as a result of the late week cold front. (Figure 2). Normal precipitation for the first week of February only
ranges from around one inch along the Ohio River to less than 0.25 inch
in northwestern Minnesota. Actual precipitation amounts for
the week ranged from 0.75 to 1.00 inch in southern and western Missouri
to about 0.35 inch in northern Wisconsin and Michigan (Figure 3).
Temperatures this week were well above normal, with departures as much
as 21F above normal in the northern Midwest, but only 3F to
6F above normal in the far southern Midwest (Figure 4).
Maximum temperatures on February 5 reached the 50s as far north as Lake
Superior and northern Michigan. The increasingly mild weather
combined with clear skies rapidly ate away at the snow cover over the
central Midwest that resulted from the storm on January 29. (Figure 5) is
a sequence of visible satellite photos showing the shrinking snow cover
especially over the central Midwest during the period from February
1 to 5. At the end of the week snow on the ground was confined to
an area north of a line from St. Joseph, MO to Chicago, IL
to Detroit, MI.
High Pressure Brings Quiet Weather, Poor Air Quality
For the first half of the week high pressure lay sprawled from the northern Rockies through the Midwest (Figure 6). Light winds associated with the high combined with overnight temperature inversions over the Midwest led to worsening air quality through the fours days of the month. Figure 7 shows the sounding depicting the temperature profile over Detroit, MI on the morning of February 3. The temperature at ground level was 10F, while at 5,000 feet the temperature was 29F. The strong inversion prevented the air near the surface from mixing with air aloft and thus dispersing the pollutants. Air quality was especially poor around major metropolitan areas such as St. Louis, Milwaukee, Chicago and Detroit (Figure 8, USEPA). However, a large portion of the Midwest experienced poor air quality. Minnesota, Iowa, Wisconsin, Illinois, Indiana, and Ohio posted air quality alerts. All of these states reported an Air Quality Index (AQI) values ranging from 110 to 150, indicating unhealthy air for citizens with diminished or sensitive respiratory function. Minnepolis-St. Paul recorded an AQI of 155 during the week, the highest in 25 years. Air quality began to improve on February 4 as southwest winds increased ahead of a cold front pushing into the northern Midwest.